A robust and recyclable ionic liquid-supported copper(II) catalyst for the synthesis of 5-substituted-1H-tetrazoles using microwave irradiation

Article abstract of DOI:10.1007/s11164-019-04035-4

Abstract: A novel and robust ionic liquid-supported copper(II) catalyst has been developed and explored for the efficient synthesis of 5-substituted-1H-tetrazoles using microwave irradiation. The ionic liquid-supported catalyst facilitated the efficient isolation of tetrazole products with high purity by simple extraction with organic solvent. Recovered ionic liquid-supported copper(II) catalyst could be recycled for three times for the synthesis of tetrazole products with high purity. This synthetic protocol offers a very clean, convenient, and microwave-assisted environment-friendly method for the efficient synthesis of 5-substituted-1H-tetrazoles with high yield. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-019-04035-4

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-04035-4
A robust and recyclable ionic liquid‑supported copper(II)
catalyst for the synthesis of 5‑substituted‑1H‑tetrazoles
using microwave irradiation
R. D. Padmaja1 · Kaushik Chanda1
Received: 3 September 2019 / Accepted: 26 October 2019
© Springer Nature B.V. 2019
Abstract
A novel and robust ionic liquid-supported copper(II) catalyst has been developed
and explored for the efcient synthesis of 5-substituted-1H-tetrazoles using micro-
wave irradiation. The ionic liquid-supported catalyst facilitated the efcient isola-
tion of tetrazole products with high purity by simple extraction with organic solvent.
Recovered ionic liquid-supported copper(II) catalyst could be recycled for three
times for the synthesis of tetrazole products with high purity. This synthetic pro-
tocol ofers a very clean, convenient, and microwave-assisted environment-friendly
method for the efcient synthesis of 5-substituted-1H-tetrazoles with high yield.
Graphic abstract
Keywords 1H-tetrazoles · (3+2) cycloaddition · Microwave irradiation · Ionic
liquid
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-04035-4) contains supplementary material, which is available to authorized users.
*
Kaushik Chanda
chandakaushik1@gmail.com
1
Department of Chemistry, School of Advanced Science, Vellore Institute of Technology,
Vellore 632014, India
Vol.:(0123456789)
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R. D. Padmaja, K. Chanda
Introduction
Because of the possession of abundant biological activities such as pharmaceu-
ticals and agrochemicals, nitrogen-containing heterocyclic compounds are get-
ting more and more attention from synthetic chemists [1–3]. Densely substituted
tetrazole derivatives are one of the most important classes of compounds widely
found in natural products and pharmaceutical molecules owing to their profound
bioactivities [4, 5]. Further, this privileged moiety is known as a bioisosteric sub-
stituent of carboxylic acid in developing bioactive products as shown in Fig. 1
[6]. Functions of these molecules are commonly found in natural products as well
as in many commercial drugs. Hossain et al. [7] isolated and reported the X-ray
crystal structure of 6-azidotetrazolo[5,1-a]phthalazine (A), a new toxic metabo-
lite of the dinofagellate Gymnodinium breve. There are tetrazole-containing drug
molecules such as losartan (B) used for antihypertensive activity, tomelukast
(C) used for antileukotriene antiasthmatic property, and selective antagonists of
NMDA and AMPA receptors (D, E) used for the treatment of schizophrenia [8,
9].
These nitrogen-containing molecules have noted robustness facilitating their
applications in coordination chemistry, in the photographic industry, or as com-
ponents of special explosives.
In most of the existing studies on 5-substituted-1H-tetrazole derivatives, these
compounds have been synthesized following (3 + 2) cycloaddition of organic
nitriles with sodium azides which was initiated frst time by Sharpless and his
coworkers in 2001 [10]. Various catalysts utilized for this purpose include both
metals and nonmetal catalysts such as Al3+ [11], Pd0 [12], Zn2+ [13], Yb3+ [14],
Fe3+–Si4+ [15], Ag+ [16], Ag0NPs [17], Co2+ [18], Cu2+ [19] Et3N.HCl [20], I2
or silica-supported NaHSO4 [21], TBAF [22], chitosan-derived magnetic ionic
liquid [23], CAN-HY zeolite [24], activated fuller earth [25], B(C6F5)3 [26],
4-(N,N-dimethylamino)pyridinium acetate [27], Cuttlebone [28], and CAES [29].
Recently, we have demonstrated the synthesis of 5-substituted-1H-tetrazole deriv-
atives using both homogeneous Cu(I) catalyst in water and heterogeneous CuO
nanoparticles under microwave irradiations [30, 31]. More importantly, most of
these methods require the use of hazardous hydrazoic acid and high catalyst load-
ing along with harsh reaction conditions and numerous of them are either not
eco-friendly or source of severe environmental pollution. As a result, still there is
Fig. 1 Examples of bioactive molecules containing the tetrazole core structure
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A robust and recyclable ionic liquid-supported copper(II)…
a need to develop a more efcient, simple, milder, and high-yield protocol for the
synthesis of 5-substituted-1H-tetrazole derivatives.
In the last decade, to suite for a particular chemical transformation, task-specifc
ionic liquids (TSILs) with functional groups tethered covalently to the either cat-
ion or anion have been developed [32, 33]. Since then, many low molecular weight
TSILs have been used as supported catalysts, reagents, and supported substrates for
organic synthesis [34, 35]. By tuning the cation and anion, the entire property of
an ionic liquid can be modifed which includes thermal stability, vapor pressure,
and the solvating ability. Ionic liquid-supported catalysts have the ability to easily
separate from the substrate and products after completion of the reaction by add-
ing a less polar organic solvent in which the ionic liquid anchored species is not
soluble. At last, the recovered ionic liquid-supported catalyst can be regenerated
or reused further to obtain the desired product. Due to these unique characteristics,
ionic liquid-supported catalysts are known as excellent heterogeneous catalysts in an
efort to stop the environmental degradation and the answer to sustainable develop-
ment which forms the core of synthetic organic chemistry research. Previously, we
have shown the use of task-specifc ionic liquids (TSILs) as soluble support for the
numerous synthetic transformations to obtain privileged heterocycles under micro-
wave irradiations [35–38]. Moreover, the use of microwave (MW) technology in
chemical synthesis is well established. It greatly accelerates the chemical transfor-
mation with much higher yields, less waste generation, diferent reaction selectivi-
ties, and mild reaction profle compared to the conventional heating system [39].
As part of our recent report on the diversity-oriented synthesis of bioactive hete-
rocycles [40–44], herein, we wish to report a novel and robust ionic liquid-supported
Cu(II)-catalyzed synthesis of 5-substituted 1H-tetrazole derivatives under micro-
wave irradiation. The synthetic manipulation involves the recyclable ion-supported
Cu(II)-catalyzed microwave-assisted (3+2) dipolar cycloaddition of organic nitriles
with NaN3 to obtain the 5-substituted 1H-tetrazole derivatives in excellent yields.
Results and discussion
At the onset of our study, the recyclable ionic liquid-supported copper(II) catalyst
5 was synthesized in aqueous solutions in three steps as depicted in Scheme 1. The
present strategy toward the synthesis of ionic liquid support 4 equipped with car-
boxyl group linker, 1-(1-carboxymethyl)-3-methylimidazolium tetrafuoroborate
([carbmmim][BF4]), was prepared in two steps. The frst step involved the reaction
of N-methyl imidazole 1 with 2-chloroacetic acid 2 in refuxing CH3CN solution to
obtain the intermediate 3 which underwent anion exchange reaction with NaBF4 to
obtain the carboxyl group-functionalized ionic liquid support 4.
The next step of the catalyst synthesis involved the reaction of carboxyl group-
functionalized ionic liquid support 4 with Cu(OAc)2 in refuxing water medium
for 8 h to obtain the ionic liquid-supported copper(II) catalyst 5 as pale blue
powder with excellent yield. The catalyst was characterized by nuclear magnetic
resonance (1H & 13C NMR), IR, mass spectroscopy, and atomic absorption spec-
troscopic measurement. Figure 1 demonstrates the quantitative catalyst formation
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R. D. Padmaja, K. Chanda
Scheme 1 Synthesis of ionic liquid-supported Cu(II) catalyst 5 in aqueous medium
1
by H, 13C NMR, IR, and mass spectroscopy in each step with an attached ionic
liquid (IL) tag. It has been found that the three protons Ha, Hb, and Hc of free
IL-tag appeared at 8.67, 7.41, and 7.40 ppm, respectively, in spectra A, whereas
the N-CHd2 and N-CH3 protons appeared at 5.03 and 3.68 ppm in Fig. 2. How-
ever, the chemical shift of these three protons was shifted to more upfeld portion
upon reaction with Cu(OAc)2. Similarly, the 13C spectrum of ionic liquid support
4 and catalyst 5 clearly shows all of the carbon present in the structures. Further,
Fourier transform infrared spectroscopy (FT-IR) measurements were carried out
on ionic liquid support 4 and Cu(II) catalyst 5. The C=O stretching frequency
which is observed at 1745 cm−1 in ionic liquid support 4 shifted to 1654 cm−1
in ion-bound Cu(II) catalyst 5 due to the electronic reorganization of the double
Fig. 2 1H NMR, IR, and mass spectra of ionic liquid-supported Cu(II) catalyst 5
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A robust and recyclable ionic liquid-supported copper(II)…
bond. The band at around 3600 cm−1 is attributed to the –OH stretching vibration,
which comes from the surface-adsorbed H2O. Moreover, the ionic liquid support
4 and Cu(II) catalyst 5 could be confrmed with mass spectra (MS) which are
shown in Fig. 2. The copper content of Cu(II) catalyst 5 was found to be 8.5%, on
the basis of atomic absorption spectroscopy analysis.
Initially, in order to optimize the reaction condition, we have chosen the reac-
tion of benzonitrile 6a with NaN3 as model reaction in Scheme 2. The catalytic
activity of ionic liquid-supported Cu(II) catalyst 5 was examined in the reaction
between benzonitrile 6a and NaN3 under neat condition at 100 °C for 6 h, and the
reaction did not yield any 5-phenyl-1H-tetrazole 7a product (Table 1, entry 1).
Unfortunately, the reaction did not aford any cyclized product using H2O-IPA at
100 °C or refuxing THF (Table 1, entries 2, 3), whereas the reaction aforded the
cyclized product 7a in 40% yield in refuxing CH3CN solution (Table 1, entry 4).
In an efort to increase the yield, the same reaction was repeated in refuxing
MeOH solution resulting in the formation of cyclized product 7a in 60% yield
(Table 1, entry 5). However, the yield of the product 7a did not increase sig-
nifcantly upon prolonging the reaction time to 8 h (Table 1, entry 6). Next we
turned our attention for polar aprotic solvents such as DMSO at 130 °C for 6 h,
which was proved to be efcient for this reaction with 80% yield of product 7a
(Table 1, entry 7). Apart from DMSO, when the same reaction was carried out
in DMF solvent for 6 h at 130 °C, 5-phenyl-1H-tetrazole 7a was formed in 90%
yield (Table 1, entry 8). However, to improve the synthetic protocol, the same set
of reaction was performed under microwave irradiation for 20 min using DMF
as solvent at 130 °C for 320 W to obtain the desired product 7a in 95% yield
(Table 1, entry 9). It is interesting to observe that the yield of the product 7a did
not increase with increasing the catalyst loading up to 15 mol % under microwave
irradiation for 20 min (Table 1, entry 10). So the optimum catalyst loading of
10 mol % was sufcient for obtaining the maximum yield of 95% for 5-phenyl-
1H-tetrazole 7a derivative.
Table 1 provides the complete optimization study for the (3 + 2) cycloaddition
reaction to obtain the 5-phenyl-1H-tetrazole 7a. Subsequently, under metal-free
condition the same set of reaction did not yield any cyclized product 7a (Table 1,
entry 11). High-power microwave irradiation allowed us to reduce the reaction
time to 20 min compared to several hours under diferent refuxing conditions.
Scheme 1 demonstrates the ionic liquid-supported Cu(II) catalyzed synthesis of
Scheme 2 Ionic liquid-supported Cu(II) catalyzed microwave-assisted synthesis of 5-substituted
1H-tetrazoles
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R. D. Padmaja, K. Chanda
Table 1 Optimization of the reaction condition for the synthesis of 5-phenyl-1H-tetrazole catalyzed by
ionic liquid-supported Cu(II) catalyst
Reaction was performed using 1a (1 mmol), NaN3 (1.2 mmol)
aMicrowave reactions were carried out in microwave model no. CATA R (Catalyst System, Pune) using
power 320 W
bYield of the isolated product
5-substituted 1H-tetrazole derivatives which involves the (3 + 2) cycloaddition
reaction under microwave irradiation.
After completion of the reaction, the reaction mixtures were precipitated with
cold ether and fltered through fritted funnel to obtain the ionic liquid-supported
Cu(II) catalyst. Upon treatment of the supernatant liquid with dil HCl, followed
by organic workup, resulted substituted 1H-tetrazole derivatives in excellent
yields.
Encouraged by the efcient synthetic approach for the desired cycloaddition
as described above, we investigated the reaction using a variety of substituted
nitriles and NaN3 as substrates under the same condition. The corresponding
5-substituted-1H-tetrazoles 7 were isolated in excellent yields under the present
reaction condition as depicted in Table 2.
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A robust and recyclable ionic liquid-supported copper(II)…
Table 2 Ionic liquid-supported Cu(II) catalyzed microwave-assisted synthesis of 5-substituted 1H-tetra-
zoles
Reaction conditions 6 (1 mmol), NaN3 (1.5 mmol), ILCu(II) catalyst (10 mol%), DMF (5 mL), 130°C
aYield of the isolated product 7
The overall reaction time is 20–25 min for the completion of the reaction. The
corresponding 5-substituted 1H-tetrazole derivatives were obtained with excel-
lent yields after a simple workup involving the precipitation of the reaction mix-
ture with cold ether for the fltration of the catalyst, acidic treatment, and sol-
vent evaporation. Finally, column chromatography purifcation of crude products
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R. D. Padmaja, K. Chanda
1
followed by spectroscopic characterization using HNMR, 13CNMR, and MS
spectroscopy confrmed the formation of desired products. Reaction yields were
varied with the substituents present in the aromatic rings. Due to the electro-
philic nature of the nitrile moiety attached to the aromatic rings, unsubstituted
or electron-withdrawing substituents reacted nicely to obtain the corresponding
1H-tetrazole derivatives, whereas electron-donating substituents resulted with
lower yields. Unlike aliphatic nitriles where there is no product formation, the
benzylic nitrile reacted nicely to obtain the corresponding 1H-tetrazole in high
yield. However, in an efort to diversify the synthetic methodology, the cycload-
dition reaction was performed with 2,6-dimethyl benzonitrile under the same
synthetic condition. But the reaction failed to obtain the desired disubstituted
tetrazole owing to the steric nature of the substituents present in the benzonitrile.
Even the precursor like organic azides failed to produce the desired 1H-tetrazole
under the optimized reaction condition.
A suggested mechanistic pathway for the (3 + 2) cycloaddition is shown where
the ionic liquid-supported Cu(II) catalyst activated the substituted nitriles 6 fol-
lowed by the addition of NaN3 resulted the formation of cyclic intermediate.
Upon acidic treatment of the cyclic intermediate resulted the generation of 5-sub-
stituted-1H-tetrazoles 7 in excellent yields. Further, the recyclability of the ionic
liquid-supported Cu(II) catalyst 5 was also surveyed and is depicted in Fig. 3.
After a reaction, the reaction mixture was precipitated with cold ether to flter
the ionic liquid-supported Cu(II) catalyst through a fritted funnel and was suc-
cessively washed with cold ether two to three times. After being dried, the cata-
lyst can be reused directly without further purifcation. The ion-supported Cu(II)
catalyst 5 can be recovered, recycled, reused for two consecutive cycles without
the loss of catalytic activity.
In summary, ionic liquid-supported Cu(II) catalyst was successfully syn-
thesized and applied as an efcient catalyst in the preparation of library of the
5-substituted-1H-tetrazoles in excellent yields under microwave irradiation. The
catalyst is thermally stable, green, easy to prepare, and devoid of any drawback
associated with homogeneous catalysts. In addition, it can be easily separated
from the reaction mixture with cold ether and recovered up to three cycles with-
out the loss of catalytic activity and reaction yield.
Fig. 3 Ionic liquid-supported Cu(II) catalyzed [3+2] cycloaddition reactions of benzonitrile 6a with
sodium azide
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A robust and recyclable ionic liquid-supported copper(II)…
Experimental section
General procedure
Unless otherwise indicated, all common reagents and solvents were used as
obtained from commercial suppliers without further purifcation. 1H NMR
(400 MHz) and 13C NMR (100 MHz) were recorded with a Bruker DRX600
spectrometer. Chemical shifts are reported in ppm relative to the internal solvent
peak (δ = 7.26 and 77.0 ppm, respectively, for CDCl3). Coupling constants, J, are
given in Hz. Multiplicities of peaks are given as: d (doublet), m (multiplet), s
(singlet), and t (triplet). GC mass spectra were recorded on a PerkinElmer MS.
Microwave-assisted reactions were carried out in a Catalyst Scientifc Microwave
oven system (model no: CATA R (Catalyst System, Pune) operating at 2450 MHz
equipped with glass vial extension by a condenser which was used for perform-
ing the reaction. The microwave was equipped with a temperature control system
(external probe).
General procedure for the synthesis of ionic liquid support 4
In a round-bottomed fask, a mixture of N-methyl imidazole 1 (1.0 equiv) and
2-chloro acetic acid 2 (1.1 equiv) was added to a 5 mL CH3CN solution and
refuxed at 80 °C for 10 h. After completion of the reaction as determined by
TLC, to the same reaction mixture was added NaBF4 (1.2 equiv) and continue
stirring for at the same temperature for 10 h. After completion, the precipitated
NaCl was fltered, and CH3CN was removed under reduced pressure and washed
several times with ether followed by drying to obtain the viscous pale yellow liq-
uid as ionic liquid support 4.
Ionic liquid-supported Cu(II) catalyst 5. In a round-bottomed fask, a mixture
of carboxyl group-functionalized ionic liquid support 4 (1.0 equiv) and Cu(OAc)2
(0.5 equiv) was added to a 10 mL H2O solution and refuxed for 8 h. After com-
pletion of the reaction, the precipitated blue colored solid was fltered and washed
several times with H2O to obtain the ionic liquid-supported catalyst 5.
General procedure for the synthesis of 5‑substituted‑1H‑tetrazoles (7a) In a round-
bottomed fask, a mixture of benzonitrile 6a (0.052 g, 0.50 mmol, 1.0 equiv) and
NaN3 (0.048 g, 0.75 mmol, 1.5 equiv) was added to a 5 mL DMF containing
10 mol% of ionic liquid-supported Cu(II) catalyst 5. The reaction mixture was irra-
diated under microwave heating at 320 W for 20 min at 130 °C. The progress of the
reaction was monitored by TLC. After completion of the reaction, the mixture was
precipitated with cold ether and fltered through a fritted funnel to remove the cata-
lyst. The fltrate was acidifed with 5 N HCl (10 ml) to neutralize the product and
extracted with ether (2×10 ml). The combined organic layer was dried over anhy-
drous MgSO4. The combined fltrate was subjected to evaporation to obtain the pure
compound 5-phenyl-1H-tetrazoles 7a as the product.
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R. D. Padmaja, K. Chanda
1‑(1‑carboxymethyl)‑3‑methylimidazolium tetrafuoroborate ([Carbmmim][BF4])
(4) Pale yellow liquid. 1H NMR (400 MHz, D2O) δ 8.67 (s, 1H), 7.41(s, 1H), 7.41
(s, 1H), 5.03 (s, 2H), and 3.68 (s, 3H); 13C NMR (100 MHz, D2O) δ 172.4, 139.8,
125.6, 124.0, 52.2, 38.4; IR (KBr, cm−1): 3574, 3166, 1739, 1579, 1409, 1176 cm−1,
−
MS (FAB) m/z: 141 (M-BF4 ).
Ionic liquid‑supported Cu(II) catalyst (5) Light blue solid. 1H NMR (400 MHz, D2O)
δ 8.44 (s, 1H), 7.40 (s, 1H), 7.23 (s, 1H), and 3.89 (s, 3H); 13C NMR (100 MHz,
D2O) δ 140.4, 126.4, 126.1, 38.4, 32.1; IR (KBr, cm−1): 3519, 3169, 1654,
−
1570 cm-1, MS (FAB) m/z: 343 (M-BF4 ).
Acknowledgements The authors thank the chancellor and vice chancellor of VIT University for provid-
ing opportunity to carry out this study. Further the authors wish to thank the management of this univer-
sity for providing seed money as research grant. Kaushik Chanda thanks CSIR-Govt of India for funding
through Grant no 01(2913)/17/EMR-II.
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